This invention relates to interferometers, and more particularly to the problem of maintaining the proper orientation of one path with respect to another.
Interferometers are coming into widespread use under such operating conditions that it is becoming increasingly difficult to maintain the precision required for the application. For example, a high-resolution Fourier interference spectrometer capable of operating in severe environments is sometimes required to provide continuous coverage of the wavelength interval of 2000 to 8000 cm.sup.-1 with a resolution of 0.3 cm.sup.-1 for airborne meteorological measurements, and planetary observations from outside the earth's atmosphere. The heart of the instrument is the interferometer.
In a typical interference spectrometer, a moving reflecting element steps incrementally under control of a monochromatic reference source and remains stationary during a data-taking period. Although this step-method of scanning is somewhat more complex than constant-speed scanning, it eliminates errors due to driving speed fluctuations. That is an important consideration in a high-vibration environment, because the optical path difference between the two beams is known, and constant, at every instant of the data-taking period. The problem is maintaining the proper orientation of one reflecting surface with respect to another as the one reflecting surface is driven from one position to the other.
In a Michelson-type interferometer one plane mirror is fixed in position at a 45.degree. angle with respect to a beam splitter, and the other plane mirror is oriented at a 90.degree. angle with respect to the one mirror. As the second mirror is adjusted in position by a linear motion actuator to vary the difference in the length of the two paths, or arms, of the interferometer, the 90.degree. orientation of the second mirror may deviate over the full travel of the linear motion actuator, particularly if that full length is of the order of several inches.
A corresponding problem arises in other types of interferometers such as those utilizing cat's-eye retroreflectors in place of plane mirrors. Light coming through either arm of the instrument is reflected by a small plane mirror on the axis of the cat's-eye reflector. Thus, a change is made in the optical path difference of four times the motion of the mirror relative to its cat's-eye reflector, instead of twice, as in the case of moving a mirror in a Michelson-type interferometer. It is possible to maintain the small retroreflector mirror fixed in position relative to its reflector, and to move the entire retroreflector with a linear motion actuator to change the optical path difference, but it is then a problem to maintain the axis of the moving retroreflector fixed in space. Tilting the axis of the retroreflector may have the same deleterious effect as tilting a plane mirror in a Michelson-type interferometer, depending upon the optical configuration of the instrument. Accordingly the purpose of the present invention is to provide for automatically correcting any angular tilt of the variable arm of an interferometer, and to automatically make a compensating adjustment in the optical configuration of the instrument.